理解此websocket框架解析代码

import struct

def parse_websocket_frame(frame):
    fin = (frame[0] & 0x80) != 0
    opcode = frame[0] & 0x0F
    mask = (frame[1] & 0x80) != 0
    payload_length = frame[1] & 0x7F

    if payload_length == 126:
        payload_length = struct.unpack(">H", frame[2:4])[0]
        mask_offset = 4
    elif payload_length == 127:
        payload_length = struct.unpack(">Q", frame[2:10])[0]
        mask_offset = 10
    else:
        mask_offset = 2

    if mask:
        masking_key = frame[mask_offset:mask_offset+4]
        data_start = mask_offset + 4
    else:
        masking_key = None
        data_start = mask_offset

    data = frame[data_start:data_start+payload_length]

    if mask:
        unmasked_data = bytearray()
        for i in range(len(data)):
            unmasked_data.append(data[i] ^ masking_key[i % 4])
        data = bytes(unmasked_data)

    return fin, opcode, data